Very low in situ permeability gas reservoirs (Kgas<0.1mD) are very common and represent a major portion of the current exploitation market for unconventional gas production. Many of these reservoirs exist regionally in Canada and the United States and also on a worldwide basis. A considerable fraction of these formations appear to exist in a state of noncapillary equilibrium (abnormally low initial water saturation given the pore geometry and capillary pressure characteristics of the rock). These reservoirs have many unique challenges associated with the drilling and completion practices required in order to obtain economic production rates. Formation damage mechanisms affecting these very low permeability gas reservoirs, with a particular emphasis on relative permeability and capillary pressure effects (phase trapping) will be discussed in this article. Examples of reservoirs prone to these types of problems will be reviewed, and techniques which can be used to minimize the impact of formation damage on the productivity of tight gas reservoirs of this type will be presented.
FIGURE 2 Aqueous phase trapping in an oil or gas reservoirrelative permeability relations. FIGURE 3: Illustration of variation of Sw irr with permeability and capillary geometry.FIGURE 4: Illustration of effect of relative permeability curve configuration on severity of aqueous or hydrocarbon phase trapping. The Journal of Canadian Petroleum TechnologyFIGURE 6: Illustration of apt correlation for preliminary diagnosis of aqueous phase trap problems.
Many hydrocarbon bearing reservoirs exhibit the potential for significant productivity reductions due to adverse relative per-meability effects associated with the retention of invaded aque-ous fluids. Those fluids could include water-based drilling mud filtrates, completion fluids, fracture fluids, workover fluids, kill fluids or stimulation fluids (including spent acid). This paper i entities potential mechanisms behind phase trapping and identifies particular reservoir types which tend to be susceptible o this type of formation damage, most notably low initial wat saturation gas reservoirs and strongly oil-wet oil reservoirs. Laboratory techniques to investigate the severity of aqueous trapping and various remedial techniques are described, and I o field case studies illustrating the potential for permeability impairment due to invasive aqueous trapping are presented. One case study describes a series of wells completed in the Paddy fo ation and the second in the Cadomin formation in the Deep Basin area of central Alberta (both gas producing zones). Laboratory case studies documenting the phenomenon of aqueous phase trapping in strongly oil-wet porous media are also presented. IntroductionOil and gas bearing formations are potentiallv susceptible to many different types of formation daiiiage(I 6). In this paper we are exclusively concerned with damage associated with aqueous phase trapping (or water trapping or blocking as it is often referred to). To understand the concept of aqueous phase trappin , it is essential to differentiate between the concept of initiat (often referred to as connate) aqueous phase saturation (S,i) and irre-ducible aqueous phase saturation a) Initial aqueous phase saturation is the initial average frac-tional portion of the pore space which is occupied by watei-. The value of the initial aqueous phase saturation is con-trolled by numerous factors, includina reservoir geology, depositional history, temperature, wettability, height above free water contact and pore size distribution. The key point to differentiate in this area is that the initial aqueous phase saturation is not necessarily, and often is not, equal to the irreducible aqueous phase saturation and can be either higher or lower than the irreducible saturation. It is in the secoii(i case, where the S,i is less than S,i,.,., where productivity November 1994, Volume 33, No. 9 k. k, i@Kiw -0 85 0 8 _ ia 0 6 > 0 4 Ig @SM,, -0 85 0 2 k. ., kg = 0 03 I,,@d..ibl. S. 10%@ (,5%) 65% S@, 00 0 0 0 2 0,4 0 6 0.8 Water Saturation FL(,URE 1: Mechanism of aqueous phase trapping. reductions due to phase trapping can occur. h ) Irreducible aqueous saturation represents that saturation which is forced to exist in the reservoir by capillary mechan-ics. Once again, the value of the Swi, is determined by para-meters such as the reservoir morphology, pore size distribu-tion, poi,e throat size distribution, wettability, surface rough-ness, etc. We often obtain estimates of Swi, through the use of air-brine or air-mercury capillary pressure tests. ...
Solids precipitation from reservoir crudes has been recognized as a serious detriment in numerous oil systems world wide. Precipitation may result in in situ permeability reductions as well as contributing to serious plugging problems in surface facilities. The latter can be treated with periodic cleaning techniques (xylene or toluene injection and/or mechanical treatment) but, rather than concentrate on remedies, prevention would be a preferable approach. This paper describes experimentally determined solids precipitation trends as a function of hydrocarbon solvents added. Techniques for the measurement of solids precipitation at reservoir conditions are reviewed and the implementation of a high-pressure high temperatureLASER cell is described. Three solids precipitation models were developed as companions to the experimental program. The models co1l.Sisted of an EOS methodology to calculate vapour-liquid equilibria but then three different models were evaluated for the liquid-solid interaction. The first was a pure component solid phase fugacity correlation, the second a regular-solution-theory multi component model found in the literature and the third was the same multi component solid model but was modified for pressure and temperature influences based on theoretical constructs. A comparison of the three models is included with quantitative comparisons between some experimental data sets and Theory. Future direction of this project is also discussed. Introduction Solids precipitation from reservoir fluids has been a problem for many years. Katz and Beau(1) were some of the first to begin researching the problem and many worthy groups have continued to study the phenomenon. The range of research efforts has been broad but there has been basically two approaches: the first has been associated with clean-up methods such as improved means of unplugging Lines or solids dissolution, whereas the second has been interested in mitigating solids precipitation before it happens, One approach is remedial and the other preventative and many centres are now applying a combination of the two. The significance of the solids precipitation has been established by many and Leontaritis and Mansoori(2) have reported the technical and economic impact with respect to numerous oils throughout the world. The unanimous conclusions of all who have researched the phenomenon indicate that the economic impact, due to reduced productivity, is substantial and requires efficient resolution but that the problem is very complex and has to date resisted an adequate description. Experimentation has yielded a number of general trends but their quantification appears to be intractable as yet. Complex molecular-scale theories incorporating resin-asphaltene interaction and electrokinetic phenomena are beyond present analysis. There are, however applied methods which can be used in semi-quantitative analysis of the solids precipitation problem. Over the last five years Hycal Energy Research Laboratories Ltd. has performed experimental and theoretical research into solids precipitation and considerable insight into the phenomena has been acquired. This paper describes the experimentation done to determine important trends and provides an overview of three theoretical equation-of-state models developed. Previous Work Initial research into solids precipitation concentrated on structure and characterization of the compounds.
This pepar was prepared for c.rea~tatim? at UM Gas Tednolcgy Conferanca held in Calgaw, A2mrla, Canada, 28A@ -1 My +W& l&ls papar was seladad for preaamtatti by an SPE Pmgmm Commiftaa folfowrng Mew of Information mnlahad h an abatmct aubmtiad by the milk{(s). Cantants d the paper, as prmented, have no! bear7 ra4Wed by me SOdeIy 01 PeMOla+JmEngineafa and are stijacted to corradon by fhe aumor(s). The material, as weaiinted, does nd neceaaarity raflad any pmttii d lhe SocWY d P8tmfeum Engineers, M dflcara. or mambera Papars Premntad at SPE meetings are sub+acl 10 puMaflcm raviaw by Edtorial Committeaa of IIW Sdily of Petroleum Engrneera. Perrnlaeicm 10 mpy is reatrb'xad to an abstrad of not mora than 300 Words Ihmtmtlons may not ba cqiad l?M qbstrad ahodd contain conspkuous admowladgemenl 01where and by tiom the pap-ar was praaenmd. Write Librarian, SPE, AbstractAs the industry seeks to increasingly exploit reserves of natural gas contained in low permeability intercrystalline sandstone and carbonate formations (-QO mD in permeability) many questions have arisen as to the optimum practices to drill and complete horizontal and vertical wells in these systems as well as the best techniques to hydraulic or acid fracture these formations to obtain economic production rates. This paper provides a summary of recent work which has been conducted in the diagnosis and remediation of problems associated with tight gas reservoirs. Information on the importance of reservoir quality assessment and initial saturation determination is presented as well as a detailed discussion of common damage mechanisms which can affect the productivity of tight gas formations.These include fluid retention problems, adverse rock-fluid and fluid-fluid interactions, counter-cument imbibition effect-s during underbalanced drilling, glazing and mashing, condensate dropout and entrainment from rich gases, fines mobilization and solids precipitation. The impact of these problems during drilling, completion, workover and kill operations is reviewed and suggestions presented for the prevention and potential remediation of these problems.
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